1. Field of the Invention
The present invention relates to a vibration driven motor in which a voltage is applied to an electromechanical energy conversion element and which is driven by a travelling vibration wave generated in a vibration member.
2. Related Background Art
The general principle of a vibration driven motor utilizing a travelling vibration wave is as follows. A member comprising two groups of piezo-electric elements circumferentially arranged and secured to one surface of a ring-like resilient member made of a resilient material having a full length integer times as great as a certain length .lambda. is used as a vibration member (stator). These piezo-electric elements in each group are arranged at a pitch of .lambda./2 and so as to assume opposite expansion polarities alternately and in such a manner that between the two groups, there is a positional deviation of an odd number times of .lambda./4. Electrode film is provided on the two groups of piezo-electric elements. If an AC voltage is applied to only one group (hereinafter referred to A phase), the vibration member becomes such that a standing wave (wavelength .lambda.) of such bending vibration that the central point of each piezo-electric element in that one group and points at intervals of .lambda./2 therefrom are the positions of antinodes, and that the central point between the respective positions of antinodes is the position of a node, is generated over the entire circumference of the resilient member. If an AC voltage is applied to only the other group (hereinafter referred to as B phase), a standing wave is likewise generated, but the positions of the antinodes and node thereof deviate therefrom by .lambda./4 relative to the standing wave of A phase. If AC voltages equal in frequency and having a time phase difference of 90.degree. therebetween are applied to A and B phases at one time, the standing waves of the two phases are combined and, as a result, a travelling wave (wavelength .lambda.) of bending vibration vibrating in the circumferential direction is generated in the resilient member. At this time, each point on the resilient member having a thickness effects elliptical motion. Consequently, if for example, a ring-like movable member as a rotor is brought into pressure contact with the other surface of the resilient member, this movable member receives a circumferential frictional force from the resilient member and is rotatively driven.
Also, it is usual to form a plurality of diametrical grooves circumferentially of the vibration member, and to increase the circumferential components of the elliptical motion in order to enhance motor efficiency. It has been confirmed that this is greatly effective. These grooves also have the effect of removing wear powder.
Referring to FIG. 7 of the accompanying drawings which is a schematic cross-sectional view of a vibration driven motor, the reference numeral 1 designates a vibration member comprising a ring-like resilient member 3 and a piezo-electric element 4 adhesively secured to the bottom surface of the resilient member 3, and having a number of comb-teeth 3a formed by the diametrical grooves of the resilient member 3. The reference numeral 2 denotes a movable member having a sliding material 5 attached to the outer edge portion of a ring 6, and this sliding material is brought into contact with the surface of the resilient member 3 by the pressing force of pressing means such as a spring, not shown.
On the basis of such a principle, the following is mentioned as the features of the vibration driven motor:
1) When no electric power is supplied, the motor has a holding torque and, moreover, does not cause hunting; and
2) The motor is quick in the rising and falling of rotation. (The mechanical time constant is small.)
Accordingly, the vibration driven motor can be said to be essentially suitable for highly accurate positioning. However if in the prior-art vibration driven motor shown in FIG. 7, polyimide (resin) filled with carbon fiber is disposed on the sliding material 5 forming the sliding surface of the movable member 2, and WC-Co (cermet) is disposed on the sliding surface of the resilient member 3 of the vibration member 1 to thereby construct a vibration driven motor, and positioning driving is actually effected, the accuracy of the motor is stable in the initial state, but positioning accuracy deteriorates with time.
So, when the vibration driven motor has been disassembled, it has been found that the sliding surface of the movable member 2 is in the shape of grooves and projections provided on the resilient member 3 of the vibration member 1, and level differences of the order of submicrons are circumferentially created. As a result, hitch is caused by the level difference portions and the edges of the projections of the vibration member.
Also, when the sliding surface of the movable member 2 has been examined carefully, it has been found that sliding wear is locally caused during the starting and stoppage of the motor to thereby form such level difference wear. Therefore, when as a countermeasure for the level difference wear, the resin has been disposed on the sliding surface of the vibration member 1, the WC-Co (cermet) has been disposed on the sliding surface of the movable member 2, and the positioning driving has been effected, circumferential level difference wear has not occurred.
However, in the above-described example of the prior art, the sliding surface of the vibration member 1 is softer than the sliding surface of the movable member 2. Therefore, the sliding surface (resin) of the vibration member 1 suffers from more wear, circumferential grooves are created in the sliding portion, and the discharge of wear powder becomes more difficult. Also, the inner and outer peripheral edges of the sliding portion of the movable member 2 contact with the edges of the grooves, and this has caused abnormal wear and output fluctuation.